The present invention relates to rolls which can be used in calenders or analogous machines to apply pressure to running webs of paper or textile, metallic foils or foils or synthetic plastic material. More particularly, the invention relates to improvements in means for enhancing the resistance of relatively long and heavy metallic rolls to flexure and/or other deforming or displacing stresses. Still more particularly, the invention relates to rolls of the type wherein a hollow cylindrical shell spacedly surrounds a stationary carrier in the form of a shaft, rod, bar or the like.
It is already known to guide a web of paper, textile material or the like through the nip of two rolls at least one of which has a hollow cylindrical shell whose external surface contacts the running web and whose internal surface spacedly surrounds a stationary carrier. At least one bearing element is installed in the hollow shell and has an outer surface bearing against the internal surface of the shell under the action of a hydraulic pressure generating device which operates between the carrier and the bearing element to counteract the forces which develop when the two rolls bear against the opposite sides of a running web therebetween. A hydrostatic seal is or can be provided between the outer surface of the bearing element and the internal surface of the shell so that the latter need not rub directly against the bearing element while rotating about its axis, either in response to transmission of torque from a suitable prime mover or in response to lengthwise movement of the running web. The outer surface of the bearing element normally extends well beyond the aforementioned pressure generating device, as considered in the circumferential direction of the shell, and the bearing element has a guide face which cooperates with and can move relative to a complementary guide face of the carrier to counteract forces which are applied to the roll in a direction substantially tangentially of the shell.
Rolls of the just outlined character can be used in calenders, in other ironing or smoothing machines, in pressure-applying units of papermaking, cellulose processing and printing machines, as well as in rolling mills for synthetic plastic materials, steel or the like.
German Auslegeschrift No. 1,193,792 discloses a roll wherein the pressure generating means comprises several hydraulic units and means for supplying such units with hydraulic fluid at a given pressure, i.e., the pressure of fluid supplied to each of several units is the same. The units are disposed in a row extending in parallelism with the axis of the shell, and their purpose is to prevent flexing of the shell as well as to uniformize the pressure between the external surface of the shell and the running web of flexible material which is caused to pass between the shell and the associated cooperating complementary roll. Each unit has a plunger which is rigidly secured to the bearing element and extends into a cylinder chamber which is machined into the carrier. In order to increase the area of contact (either directly or by way of the aforementioned hydrostatic seal) between the bearing element and the internal surface of the shell, the bearing element normally extends well beyond both sides of each hydraulic unit, as considered in the circumferential direction of the shell. The aforementioned hydrostatic seal comprises several recesses in the outer surface of the bearing element and means for filling the recesses with a pressurized friction-reducing medium, e.g., oil.
In a machine which employs rolls of the just outlined character, the shell is often subjected to the action of pronounced forces which act substantially tangentially of the shell, i.e., at right angles to the plane including the axes of the shell and of the associated complementary roll. Such tangential forces develop primarily as a result of frictional engagement between the external surface of the shell and the running web or between the external surface of the shell and the external surface of the complementary roll. The just discussed tangential forces are especially pronounced if the shell is driven by a prime mover to advance the web lengthwise and to thereby indirectly rotate the complementary roll. Additional transverse or tangential forces develop under the weight of the shell, especially when the aforementioned common plane of the axes of the shell and the complementary roll is not vertical. The transverse forces tend to or actually bend or flex the shell at right angles to the aforementioned common plane with the result that the line of contact between the shell and the running web is shifted to one side of such plane, i.e., flexing of the shell entails a change in the configuration of the nip between the shell and the complementary roll.
Any changes in the configuration of the nip are highly undesirable in many types of machines in which a roll of the just outlined character is put to use. Thus, the thickness of the web which travels between the shell and the complementary roll is not uniform, as considered in the axial direction of the shell, if the configuration of the nip is changed because this invariably entails changes in the width of the clearance between the shell and the complementary roll. Consequently, the thickness of a paper web in a calender is likely to vary in a direction from one toward the other marginal portion of the web if the shell is allowed to flex and/or to undergo other types of deformation in response to the application of stresses acting at right angles to or having components acting at right angles to the plane (hereinafter called pressure plane) which includes the axes of the shell and of the complementary roll. Such forces or stresses cause the bearing element in the interior of the shell to move sideways, i.e., at right angles to the pressure plane, except if the aforementioned cylinder chamber can hold the plunger of the hydraulic pressure generating unit against any and all movements transversely of the axis of the plunger. If the bearing element cannot yield by moving sideways, it is subjected to very pronounced tilting stresses which are applied by the rotating shell and tend to change the orientation of the bearing element with reference to an axis which is parallel to the axis of the shell. This leads to jamming of the shell and/or of the plunger in the cylinder chamber with attendant adverse influence on normal operation of the hydraulic pressure generating unit. Moreover, the outer surface of the bearing element does not conform to the internal surface of the shell so that the pressurized fluid can escape from the aforementioned recesses of the hydrostatic seal between the internal surface of the shell and the adjacent outer surface of the bearing element. This, in turn entails pronounced losses in pressurized hydraulic fluid and reduces the quality of lubrication between the surfaces of the bearing element and the shell.
German Offenlegungsschrift No. 2,259,035 discloses a modified roll wherein the shell confines a bearing element which is biased away from the stationary carrier and against the internal surface of the rotating shell by at least two hydraulic pressure generating devices which are spaced apart, as considered in the axial direction of the shell. This publication further discloses the possibility of fixedly mounting the plungers of the pressure generating devices in the carrier and providing the cylinder chambers for such plungers in the stationary carrier. The diameters of surfaces surrounding the cylinder chambers exceed the diameters of the respective plungers, and the plungers are surrounded by elastic sealing rings so that each plunger has a certain freedom of tilting movement in the respective cylinder chamber. In other words, the bearing element can be tilted with reference to the carrier about an axis which is parallel to the axis of the shell. This enables the bearing element to yield to transverse forces so as to avoid jamming of plungers in the respective cylinder chambers. The just discussed roll exhibits the drawback that the shell readily yields to tangential and other forces which act thereon in a direction at right angles to the pressure plane including the axes of the shell and the complementary roll, i.e., the configuration and location of the nip between the shell and the complementary roll are unstable. Moreover, once the bearing element is tilted in response to the application of one or more transverse forces, the direction in which the pressure generating devices act to urge the bearing element against the internal surface of the shell is shifted to one side of the aformentioned pressure plane which entails additional deformation of the shell and attendant distortion of the nip between the shell and the complementary roll.
German Offenlegungsschrift No. 2,625,048 discloses a roll with several pairs of hydraulic pressure generating devices between the carrier and the bearing element in the interior of the rotating shell. The devices of each pair have axes which extend radially of the shell and are located in different planes each of which is normal to the axis of the shell. Also, the devices of each pair are mirror symmetrical to each other with reference to the pressure plane which includes the axes of the shell and of the complementary roll. Such design also fails to compensate for transverse stresses, i.e., for forces which act at right angles to the pressure plane. Moreover, the just discussed roll exhibits the drawback that the shell is likely to be flattened between the pressure generating devices of each pair, i.e., in the region where the aforementioned pressure plane intersects the shell. This, in turn, results in pronounced contact between the shell and the running web, i.e., the contact is changed from a linear contact to a surface-to-surface contact which is highly undesirable in connection with the treatment of many types of web-, strip- or tape-like materials including paper, textiles, metallic foils and plastic foils.